Heretofore, in general, there have been two known methods for modifying or toughening thermoset resins with reactive liquid polymers (hereinafter RLPs) to improve properties such as fracture energy (G.sub.Ic) and impact strength without adversely affecting the thermomechanical property retention of the resins. This reaction can be accomplished by modifying the epoxy resin end group so that the polymer product can participate in the thermoset cure. The first prior art method is the adduction or reaction of carboxyl-terminated butadiene-acrylonitrile type copolymers with epoxy resins. This reaction can be accomplished by modifying the epoxy resin end group so that the polymer product can participate in the thermoset cure. Such adducted systems can be cured in a conventional manner to give elastomer or rubber-modified thermoset resin systems having a significant enhancement in toughness, as measured by fracture energy, over their unmodified counterparts.
A second prior art method of modifying thermoset resins is by admixing a RLP into the resin. Although generally any RLP product can be mixed into any thermoset resin to give a modified thermoset resin system having the characteristic enhanced toughness, such a combination generally is not miscible. Such lack of miscibility between the RLP and the thermoset resin results in the necessity to thoroughly mix the composition prior to use, which is inconvenient and impractical for most applications.
Although thermoset resins modified by either of the two prior art methods described above display improved toughness over their unmodified counterparts, thus making them suitable for use in certain applications, such modified thermoset resins still are unsuitable for use in many applications where an even greater level of toughness is required.
U.S. Pat. No. 3,892,819 to Najvar relates to vinyl ester resins with improved impact resistance obtained by a process modification wherein up to 20 percent of the unsaturated monocarboxylic acid, which is reacted with a polyepoxide, is replaced by an equivalent amount of a liquid carboxyl terminated polydiene rubber capable of reacting with epoxy groups to form a chemically bound molecule.
U.S. Pat. No. 3,928,491 to Waters relates to a flexible crack-resistant and chemically resistant thermosetting vinyl ester resin which is produced by coreacting an epoxy resin, a carboxyl terminated elastomer and an unsaturated monocarboxylic acid, such as acrylic or methacrylic acid. The neat resin is miscible and copolymerizable with ethylenically unsaturated monomers such as styrene. A novel cast pipe utilizes the resin of the invention as an unreinforced crack-resistant inner lining.